The purpose of this study is to determine risk factors for the acquisition/colonization of respiratory pathogens and particularly Pseudomonas aeruginosa in the respiratory tract of children with cystic fibrosis (CF). The primary hypothesis to be tested is that exposures during clinic visits of young children with CF to older patients who are colonized with Pseudomonas significantly increases the risk of acquisition of this organism in early childhood. This hypothesis will be tested by cohorting clinic visits for infants without Pseudomonas in the Madison CF Center, and comparison will be made to an integrated/ interspersed population of Pseudomonas positive and negative patients in the Milwaukee CF Center. An integrated/interspersed clinic of Pseudomonas positive and negative patients is standard practice in this country. Therefore, the patients in Milwaukee will be receiving the current standard of care.